The US government demanded Monday that the international community act to prevent Iran from making nuclear weapons, with a senior official saying Tehran had already "crossed the international red line".

Washington said it would not accept any uranium enrichment by Iran, appearing to reject a Russian proposal to resolve the dispute over Iran's nuclear programme by allowing Tehran to do small-scale nuclear fuel work.

"You can't have the regime pursuing enrichment on any scale, because pursuing enrichment on any scale allows them to master the technology, complete the fuel cycle -- and then that technology can easily be applied to a clandestine programme for making nuclear weapons," State Department spokesman Tom Casey said.